How do I set file permissions when opening a file in C++?

Question

In C++, I want to open a file and set its permission, but I failed. Below is my program:

string filename="test.cnf";
ofstream ofile;
ofile.open(filename.c_str(),O_RDONLY);
ofile.close()

But I get the following error:

error: invalid conversion from 'int' to 'std::_Ios_Openmode'

error:   initializing argument 2 of 'void std::basic_ofstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]'

How to set the permission of the file to such as 644, 700?

score 7 · Answer 1 · answered Jun 08 '11 at 07:49

The option you seem to want to specify (O_RDONLY) isn't a "permission", it's an access mode. These are set implicitly according to the combinations of std::ios_base::in and std::ios_base::out: in alone results in O_RDONLY, out alone in O_WRONLY and in | out in O_RDWR.

For the permissions on a created file, the answer is, rather annoyingly, thet you can't specify them. std::filebuf::open() (which is what std::ifstream and std::ofstream ultimately call) has no options or provisions for passing in any indication of permissions to be used if it must create the file.
The only way to do this is by using your system level functions (open under Linux, CreateFile under Windows—despite the names, open may create a file, and CreateFile will open an existing file, without creating anything). Using system level open/CreateFile, however, means using system level read/ReadFile and write/WriteFile.

thanks for your answer, it is helpful. But I have to select the first person as the accept one, because he is much quickly. Sorry :-) — zhaojing, Jun 08 '11 at 09:32
@zhaojing you are supposed to select the answer for accepting that is most correct / is the best answer. It doesn't matter which came first — UpAndAdam, Oct 17 '13 at 19:36

Mihran Hovsepyan · Accepted Answer · 2011-06-08T05:44:17.823

4

There is not O_RDONLY mode for fstream. You should use one of:

ios::app
ios::binary
ios::ate
ios::in
ios::out
ios::trunc

Or their combinations (say ios::app | ios::binary). For your case you should use ios::in (maybe with ios::binary). Look at this for more details.

edited Jun 08 '11 at 05:44

answered Jun 08 '11 at 05:37

Mihran Hovsepyan

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many thanks for your quickly reply. That is very helpful for me to get clear my mistake. – zhaojing Jun 08 '11 at 09:18
This only handles the first part – UpAndAdam Oct 17 '13 at 19:34
This says nothing about the permission assigned to the file and is less than helpful. – Jon Trauntvein Aug 04 '16 at 17:30

score 2 · Answer 3 · answered Jun 08 '11 at 05:41

2

If you are on unix systems, what you need is the "setmode" or "getmode" function.

From your shell command-line, type "man setmode"

answered Jun 08 '11 at 05:41

malkia

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I don't think `setmode` or `getmode` have anything to do with the OP's question. – Billy ONeal Jun 08 '11 at 05:57
Agree with @Billy. @malkia OP have problems with `std::ofstream` but you telling him how solve the problem if it were about `FILE*`. – Mihran Hovsepyan Jun 08 '11 at 06:54
Thanks to all! Yes, my problem is related with std:ofstream. But your method give me another way, thanks! – zhaojing Jun 08 '11 at 09:19

score 1 · Answer 4 · answered Jun 08 '11 at 07:51

The second argument of fstream::open is a combination of flags chosen in ios::app, ios::binary, ios::ate, ios::in, ios::out, ios::trunc.
There is no standard C++ interface to set the access right. The implementations I know are using 0666 which is then masked by the value set by umask(2). This is also the usual behavior of unix applications (the umask is inherited from the parent and shells have a builtin umask command). My recommandation is that you do nothing and rely on the umask set by the user. If that is not applicable, temporarily change the umask in your app.

How do I set file permissions when opening a file in C++?

4 Answers4

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